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Cold Climate Sizing

I've been having a discussion with a few people in trying to size a Geo Heat pump in a cold weather climate. It seems pretty evenly split, so I figured I would pick the brain of the Geo experts of Hvac-Talk.

In a climate with more heating degree days then cooling degree days, does one size a ground source heat pump based on the heating load or the cooling load?

The particular instance that brought this up is that I was doing a print and the load came up as 75k heating and 30k cooling. So with a two-stage heat pump, do I do a 3-ton or a 6-ton unit?

I clicked he other button in the poll. This all boils down to the numbers and trade offs. I would advocate sizeing the geo to handl at least 85% of the heating load depending on the cost of electricity to run the estimated days when you need stage 3. That decision also needs to be tempered against the cost of installing the loops. If loop installation to cover the extra heating load is more than they will spend in stage 3 for the next ten years, you have your answer. In my OA we generally size for 100% of heating due to low cost of loop install. Others still size for cooling and see a more stage 3 operation. There is no one rule of thumb, every geo job is unique and must be sized and bid as such.
Eric

I've been having a discussion with a few people in trying to size a Geo Heat pump in a cold weather climate. It seems pretty evenly split, so I figured I would pick the brain of the Geo experts of Hvac-Talk.

In a climate with more heating degree days then cooling degree days, does one size a ground source heat pump based on the heating load or the cooling load?

Cooling load of course. It doesn't matter if you're in a heat dominated area!! How would anyone size an ASHP two-stage in the Northern Climates?? The same way!!! I've seen my competitor's do it the other way, and lo and behold, uncontrollable humidity, feels clammy and the duct systems are usually undersized, I marvel at these installs..

The particular instance that brought this up is that I was doing a print and the load came up as 75k heating and 30k cooling. So with a two-stage heat pump, do I do a 3-ton or a 6-ton unit?

Neither, a 3 or 6 ton...
A two-stage 3 ton is not big enough for heating and second stage runs more than 5% for cooling..
A two-stage 6 ton is okay for heating but to big for cooling; especially if the first-stage runs more than 100% in the cooling mode..
Keep the first-stage (cooling mode) less than 100% capacity of the load and it should be okay.. Sometimes the heating load is met and sometimes not. Usually in this case a plenum heater makes it up..

The method you describe is 1 way to do it, but not the only way. I would assume that since we are talking about prints and a bid, it could be DESIGNED for 90% geo with the apropriate manual D. The installation of multiple smaller units would also be something I would explore.
Eric

Well I would size a 5 ton WaterFurnace variable capacity 7 series and /thread

But since you don't have that option I would personally oversize the cooling and try to meet heat load or get close to it. I would rather be slight uncomfortable a week or two out of the year then pay for propane or electric resistance supplemental 4-6 months of the year.

Just remember that the 2 stage units are really closer to 75-80% on low so if you got a 6 ton unit to handle the heat loss, on low the sucker is just under 5 tons so WAY oversized for 2.5 ton gain. You'd need a BIG duct system to move 2400 CFM and I'm guessing we aren't talking a mansion here. With a cooling capacity on low just under twice the gain, could have humidity problems if a humid climate. If a variable capacity compressor, would help solve that issue but if you had a big duct system and running cooling on a low capacity, would the cold air find its way upstairs or to a far room?

Around here, in a cold climate, that house would likely have a 3 ton, maybe 3.5 ton tops and 20kw backup. Our juice rates aren't so bad but what it would still be pretty cheap to heat with that combo.

I clicked he other button in the poll. This all boils down to the numbers and trade offs. I would advocate sizeing the geo to handl at least 85% of the heating load depending on the cost of electricity to run the estimated days when you need stage 3. That decision also needs to be tempered against the cost of installing the loops. If loop installation to cover the extra heating load is more than they will spend in stage 3 for the next ten years, you have your answer. In my OA we generally size for 100% of heating due to low cost of loop install. Others still size for cooling and see a more stage 3 operation. There is no one rule of thumb, every geo job is unique and must be sized and bid as such.
Eric

We have the same thing all the time here. If Im using electric as the back up I like to size the first stage to do as close to 100% of the cooling load, and try to get the second stage as close to 100% as I can. If I have a gas backup heating system, then I don't mind keeping the loop short and sizing closer to the cooling load with a 95% variable speed backup. My house for example. One zone is 25,000 cooling and 44,000 in heat. With a 3 ton Climatemaster Tranquality 27 I do 99.9% of the heating with the geo and don't use the electric back up very often at all. The only problem I have with humidity is I have to have the humidifier on in the heat of the summer. It actually drops the humidity down under 40% and its too dry.

We have the same thing all the time here. If Im using electric as the back up I like to size the first stage to do as close to 100% of the cooling load, and try to get the second stage as close to 100% as I can. If I have a gas backup heating system, then I don't mind keeping the loop short and sizing closer to the cooling load with a 95% variable speed backup. My house for example. One zone is 25,000 cooling and 44,000 in heat. With a 3 ton Climatemaster Tranquality 27 I do 99.9% of the heating with the geo and don't use the electric back up very often at all. The only problem I have with humidity is I have to have the humidifier on in the heat of the summer. It actually drops the humidity down under 40% and its too dry.

Have you tried increasing airflow? Bump the blower up to 450-500CFM per ton in cooling mode and see if that helps any.